Double prepositions: 'a rate of around 30%' or 'a rate around 30%'?

"The treatment has a complete necrosis rate of around 30%."

Or should the preposition "of" be deleted:

"The treatment has a complete necrosis rate around 30%."

To me both sentences mean the same, but which one is grammatically correct?

Use the preposition. Rate of is a common prepositional phrase, and around is used here as an adverb meaning "approximately":

around 5 approximately; about: software costs would be around $1,500 [NOAD]

Both versions (...rate of around 30% and rate around 30%) sound ok to my middle-American ear, but for a technical presentation I would instead use one of the following.

The treatment has a complete necrosis rate of about 30%.
The treatment has a complete necrosis rate of ca. 30%.
The treatment's complete necrosis rate is about 30%.